Micrometer

“The horizontal section in the direction of the axis of the telescope. The eye-piece ab consists of two planoconvex lenses a, b, of nearly the same focal length, and with the two convex sides facing each other. They are placed at a distance apart less than the focal length of a, so that the wires of the micrometer, which must be distinctly seen are beyond b. The eye-piece slides into the tube cd, which screws into the brass ring ef, through two openings in which the oblong frame, containing the micrometer slides, passes.” —The Encyclopedia Britannica, 1903

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